如何确保将字符串数组作为字段的结构在分配给另一个变量时遵循值语义

Question

In MSDN documentation it is mentioned that " Structs are copied on assignment. When a struct is assigned to a new variable, all the data is copied, and any modification to the new copy does not change the data for the original copy. "

I have struct which has a string array as the only field inside it.

struct MyVar
{
    private readonly string[] value;
    MyVar(string[] iVal)
    {
        value = iVal;
    }
}

When I assign one struct variable to another how to ensure the string array would be copied fully (deep copy) to assigned variable.

Answer 1

You can't do this in C# because there's no way to intercept the compiler-generated code to copy the struct's data from one to another.

The only thing you can really do is to make your struct fully immutable.

That means:

• When you create the struct, make a defensive copy of any mutable reference types that you store inside the struct (such as the string array in your example).
• Do not pass your mutable reference type objects to anything that can mutate them.
• Do not expose any mutable reference types from the struct. That would mean that you couldn't expose the string array from your struct.
• Don't do anything to mutate any reference types held in your struct. So in your example, you couldn't change the contents of your string array.

That's a lot of limitations. The safest way is to never store any mutable reference types in your struct...

Anyway, to make your struct safer you can defensively copy the string array:

struct MyVar
{
    private readonly string[] value;
    MyVar(string[] iVal)
    {
        value = (string[])iVal.Clone();
    }
}

That particular example is now safe to copy because it doesn't have any way for the string array to be mutated. But as soon as you add any mutator methods or expose the string array via a property, or pass it to anything that might mutate it, you're back to square one.

If you want to make a "manual" copy of your struct though, you can do that via serialization.

It's just that you can't do anything about:

MyVar var1 = new MyVar(test);
MyVar var2 = var1;